A liquid-crystal display (LCD) device includes liquid-crystal cells with liquid-crystal between transparent electrodes formed on opposed glass plates, or includes liquid-crystal cells with liquid-crystal between a transparent electrode on a glass substrate and a reflective electrode on a non-transparent substrate. By applying an electrical signal between the electrodes, the LCD device is capable of modulating incident light and displaying information. For obtaining contrast, it is necessary to uniformly control the orientation of the liquid-crystal molecules. In a field-effect system such as a nematic system which is designed to twist the liquid-crystal molecules by about 0.degree. to 360.degree. between the upper and lower plates, it is preferable to orient the liquid-crystal molecules parallel to the substrate surface and in a unidirectional manner.
There are many techniques to align liquid-crystals. They include stretching a polymer, rubbing it, depositing it in the form of a Langmuir Blodgett film, and exposing the film to UV radiation. There are also techniques in which particles of SiO are deposited on substrates or grooves are etched using microlithography. All such techniques have drawbacks.
The preferred technique has been to deposit an alignment film formed of polyimide on each transparent elect rode to orient the liquid-crystal, to rub or abrade the polyimide film with a gigged, flocked or velvet cloth in a desired direction and, subsequently, to clean the film before assembling the transparent substrates to form a liquid-crystal cell. The alignment film is an insulating layer whose atomic structure (i.e., molecules) is thus aligned in a predetermined direction to orient the liquid-crystal molecules in the predetermined direction.
The process of forming a polyimide film includes initially applying a wet coat of polyimide to a substrate. This can be accomplished by known printing or spinning techniques. The wet coat is then baked to form a polyimide film on the substrate. Thereafter, the film is run through a roller to even out the surface of the film. The film is then rubbed followed by a cleaning process to remove debris left by the rubbing process. Such a process requires numerous steps (as described above) and is time consuming and costly. Accordingly, there is a need to provide an alignment film that can be formed on a substrate more efficiently and cost effectively.
Once a polyimide film is deposited on a substrate, its atomic structure must be aligned in a predetermined direction to orient the liquid-crystal in a predetermined direction. As discussed above, this is typically accomplished by rubbing the film in a desired direction. The rubbing method has several drawbacks. The rubbing pressure introduces debris and requires cleaning steps and hence is not readily compatible with clean room processing. Moreover, in an LCD which uses an active, switching element array such as a TFT (thin film transistor) or MIM (metal-insulating layer-metal) element, the static electricity caused by rubbing the treated electrode plate may damage the semiconductor switching elements.
Another problem with the rubbing method is that, as the electrode plate is rubbed with a rubbing cloth, fibers and impurities contained in the cloth are attached to the alignment film, thereby degrading the LCD. This necessitates cleaning the LCD with a large quantity of cleaning solvent. The rubbing action also causes impurities and fibers to be released into the surrounding environment. Since it is necessary to perform the film deposition steps in a clean room environment, the rubbing step must be performed in a different location than the film deposition step. The requirement of a separate room for rubbing the film increases the overall manufacturing time and cost of an LCD.
One approach to improve the method of aligning an alignment film is found in U.S. Pat. No. 5,030,322 to Shimada et al, which discloses a method of etching a polyimide alignment film, using ion radiation, to orient the liquid-crystal molecules. Although the Shimada patent provides an improved alignment method over the prior art rubbing method, it does not provide a solution to the numerous steps required to deposit the alignment film, such as applying a wet coat, baking the wet coat to form the alignment film and running the alignment film through a roller.
There is a need to provide an efficient low cost method of both depositing and aligning an alignment film on a substrate of an LCD. There is also a need to provide an alignment procedure that is reliable and suitable for use in LCDs.
Accordingly, it is an object of the present invention to provide a reliable liquid-crystal display (LCD) which requires fewer steps and less cost to produce than the prior art.
It is a further object of the present invention to provide an improved alignment film which is easy to manufacture, reliable, and comparable in performance to alignment films of the prior art.